Method for non-contrast enhanced magnetic resonance angiography

ABSTRACT

A method for non-contrast enhanced magnetic resonance angiography (“MRA”) that has a short scan time and is insensitive to patient motion is provided. More particularly, the method provides significant arterial conspicuity and substantial venous signal suppression. A two-dimensional single shot acquisition is employed and timed to occur a specific time period after the occurrence of an R-wave in a contemporaneously recorded electrocardiogram. In this manner, k-space data is acquired that is substantially insensitive to variations in arterial flow velocity, or heart rate, and that further substantially suppresses unwanted venous signal in a prescribed imaging slice. Alternatively, a two-dimensional multi-shot acquisition is employed to acquire k-space data using an echo train length that is sufficiently short so as to suppress flow-related artifacts, and such that cardiac gating is not required.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-In-Part of U.S. patent applicationSer. No. 12/574,856 filed Oct. 7, 2009, which claims the benefit of U.S.Provisional patent application Ser. No. 61/169,925 filed on Apr. 16,2009.

BACKGROUND OF THE INVENTION

The field of the invention is magnetic resonance imaging (“MRI”) andsystems. More particularly, the invention relates to methods fornon-contrast enhanced magnetic resonance angiography (“MRA”).

When a substance such as human tissue is subjected to a uniform magneticfield (polarizing field B₀), the individual magnetic moments of thenuclei in the tissue attempt to align with this polarizing field, butprecess about it in random order at their characteristic Larmorfrequency. If the substance, or tissue, is subjected to a magnetic field(excitation field B₁) that is in the x-y plane and that is near theLarmor frequency, the net aligned moment, M_(z), may be rotated, or“tipped”, into the x-y plane to produce a net transverse magnetic momentM_(xy). A signal is emitted by the excited nuclei or “spins”, after theexcitation signal B₁ is terminated, and this signal may be received andprocessed to form an image.

When utilizing these “MR” signals to produce images, magnetic fieldgradients (G_(x), G_(y), and G_(z)) are employed. Typically, the regionto be imaged is scanned by a sequence of measurement cycles in whichthese gradients vary according to the particular localization methodbeing used. The resulting set of received MR signals are digitized andprocessed to reconstruct the image using one of many well knownreconstruction techniques.

The measurement cycle used to acquire each MR signal is performed underthe direction of a pulse sequence produced by a pulse sequencer.Clinically available MRI systems store a library of such pulse sequencesthat can be prescribed to meet the needs of many different clinicalapplications. Research MRI systems include a library of clinicallyproven pulse sequences and they also enable the development of new pulsesequences.

Magnetic resonance angiography (“MRA”) uses the magnetic resonancephenomenon to produce images of the human vasculature. To enhance thediagnostic capability of MRA, a contrast agent such as gadolinium can beinjected into the patient prior to the MRA scan. The trick with thiscontrast enhanced (“CE”) MRA method is to acquire the central k-spaceviews at the moment the bolus of contrast agent is flowing through thevasculature of interest. Collection of the central lines of k-spaceduring peak arterial enhancement, therefore, is key to the success of aCE-MRA exam. If the central lines of k-space are acquired prior to thearrival of contrast, severe image artifacts can limit the diagnosticinformation in the image. Alternatively, arterial images acquired afterthe passage of the peak arterial contrast are sometimes obscured by theenhancement of veins. In many anatomic regions, such as the carotid orrenal arteries, the separation between arterial and venous enhancementcan be as short as 6 seconds.

The short separation time between arterial and venous enhancementdictates the use of acquisition sequences of either low spatialresolution or very short repetition times (“TR”). Short TR acquisitionsequences severely limit the signal-to-noise ratio (“SNR”) of theacquired images relative to those exams in which a longer TR isemployed. The rapid acquisitions required by first pass CE-MRA methodsthus impose an upper limit on either spatial or temporal resolution.

Recently, a rare and serious pathology involving fibrosis of skin,joints, eyes, and internal organs referred to as nephrogenic systemicfibrosis (“NSF”) has been correlated to the administration ofgadolinium-based contrast agents to patients undergoingcontrast-enhanced MRA studies. The link between gadolinium-basedcontrast agents and NSF is described, for example, by P. Marckmann, etal., in “Nephrogenic Systemic Fibrosis: Suspected Causative Role ofGadodiamide Used for Contrast-Enhanced Magnetic Resonance Imaging,” J.Am. Soc. Nephrol., 2006; 17 (9):2359-2362. As a result of the increasedincidence of NSF, methods for MRA that do not rely on the administrationof a contrast agent to the patient have become an important field ofresearch. However, current methods for non-contrast angiography arelimited in their utility because they are sensitive to patient motion,do not consistently or accurately portray vessel anatomy in patientswith severe vascular disease, and require excessively long scan times.

While single shot acquisition methods such as two-dimensional (“2D”)balanced steady-state free precession (“bSSFP”) have the potential toreduce motion artifacts and shorten exam times, arterial conspicuity isinadequate due to high background signal. Moreover, bSSFP methods do notlend themselves to the creation of maximum intensity projection (“MIP”)angiograms. In one example, a saturation-recovery bSSFP pulse sequenceemployed for cardiac perfusion imaging following the administration of aparamagnetic contrast agent is described by W. G. Schreiber, et al., in“Dynamic Contrast-Enhanced Myocardial Perfusion Imaging UsingSaturation-Prepared TrueFISP,” JMRI, 2002; 16:641-652. However, thispulse sequence applies a spatially non-selective saturation pulse thatsuppresses the signal from blood and, thus, cannot be employed for MRA.Additionally, Schreiber's method does not provide a means fordistinguishing arteries from veins.

It is, in fact, particularly challenging to suppress venous signal witha single shot acquisition since, unlike arterial blood, venous bloodtypically flows slowly or even, for periods of time, not at all. Inaddition, the venous flow pattern is largely unpredictable, sometimesvarying with a patient's respiration cycle, cardiac cycle, or both.Consequently, it is problematic to eliminate the signals from veins withsingle shot acquisitions, since venous blood flows only a short distanceor not at all during the short scan time. Unfortunately, venous signalstend to overlap with arterial signals on projection images, therebymaking it difficult or impossible to diagnose arterial disease usingsuch methods for MRA. In addition, a robust single shot non-contrast MRAtechnique must provide an accurate depiction of arterial anatomy over awide range of flow velocities, ranging from a few centimeters per second(“cm/sec”) to more than 100 cm/sec. Moreover, the arterial anatomy mustbe depicted with sufficient arterial conspicuity to allow creation of aprojection angiogram.

Several approaches have been previously described to suppress venoussignal on non-contrast MR angiograms as follows. One method for venoussuppression has been accomplished using image subtraction. Techniqueslike fresh blood imaging (“FBI”) involve the subtraction of two imageswith different arterial signals, but identical venous signals. In thismanner, the venous, but not arterial, signals cancel with subtraction.Unlike the saturation-based methods, subtraction techniques eliminatethe signals from both stationary and moving venous spins. However, imagesubtraction doubles scan time and greatly increases the sensitivity ofthe technique to patient motion. In addition, these methods requireprior knowledge of flow velocities in order to maximize arterialconspicuity.

Another method for suppressing venous signals is to employ a T₂-weightedmagnetization preparation pulse, which diminishes signal in veins sincevenous blood has a reduced oxygen tension. However, this method isinconsistently effective since the level of venous oxygenation varieswidely and unpredictably.

Yet another method is to repeatedly apply a saturation radio frequency(“RF”) pulse just prior to the pulse sequence used for data acquisition,and to repeat this process multiple times at typical intervals of 20-200milliseconds (“ms”). However, the use of a single shot acquisition withsubsecond data acquisition time does not afford the time to repeatedlyapply a saturation RF pulse. As a result, this approach is onlyapplicable to multi-shot acquisition techniques where the data isacquired over tens of seconds to several minutes. Moreover, the repeatedapplication of RF pulses causes marked suppression of arterial signal intortuous vessels, thereby limiting the diagnostic accuracy of thesemethods.

A single shot acquisition method for MRA is described by R. Edelman, etal., in “Fast Time-of-Flight MR Angiography with Improved BackgroundSuppression,” Radiology, 1991; 179:867-870. This method requires the useof an inversion recovery preparation pulse and relies on arterial inflowduring the data acquisition period to produce arterial contrast. In thisrespect, the inversion time (“TI”) is selected solely to match thecenter lines of k-space to the “null” point for the longitudinalmagnetization of background tissue, and is not selected in order toallow for the inflow of arterial blood into the imaging slice. In otherwords, the purpose of the TI is to reduce the signal intensity ofbackground tissues.

This method suffers from several drawbacks. For example, the methodacquires data over a lengthy time period on the order of one second,thereby encompassing both systole and diastole. With this lengthy timeperiod required for data acquisition, it is not possible to synchronizeTI to the period of rapid, systolic arterial flow, nor to the period ofslow diastolic flow. Moreover, the TI employed by Edelman is too short(on the order of 75 ms) to allow for substantial arterial inflow. As aresult, most of the arterial inflow occurs during the application ofrepeated RF pulses. As described above, the repeated application of RFpulses in this manner causes marked suppression of arterial signal intortuous vessels, thereby limiting the diagnostic accuracy of suchmethods for MRA. The method also does not allow for the effectivesuppression of venous or fat signals, which are both essential toaccurately depict the arteries.

Other methods of non-contrast enhanced MRA are described, for example,by M. Katoh, et al., in “Free-Breathing Renal MR Angiography WithSteady-State Free-Precession (SSFP) and Slab-Selective Spin Inversion:Initial Results,” Kidney International, 2004; 66:1272-1278, and by Y.Yamashita, et al., in “Selective Visualization of Renal Artery UsingSSFP with Time-Spatial Labeling Inversion Pulse Non-Contrast EnhancedMRA for Patients with Renal Failure,” Proc. Intl. Soc. Mag. Reson. Med.13 (2005) p. 1715. The method described by Katoh utilizes athree-dimensional (“3D”) acquisition with a pre-inversion of the 3Dregion, while Yamashita employs two inversion pulses (one spatiallyselective and the other spatially non-selective). Each of these methodsuses inversion preparation pulses rather than saturation pulses andfurther requires the use of a 3D, rather than 2D, acquisition for MRA.Given the substantial thickness of the 3D imaging slab, inflowingunsaturated spins must travel a large distance (for example, up toseveral centimeters) to replace in-plane saturated ones. Consequently,there is poor depiction of slowly flowing arterial spins. In fact, theinversion time, TI, must be very long (on the order of 1 second) toprovide adequate inflow of even moderately fast flowing arterial spins.The long TI spans both the systolic and diastolic phases of the cardiaccycle. Given the long TI, it is problematic to synchronize dataacquisition to diastole. In addition, 3D acquisitions are tootime-consuming to permit data acquisition within a single breath-holdingperiod.

A 2D adaptation of Yamashita's “time-SLIP” acquisition is described byS. Yamada, et al., in “Visualization of Cerebrospinal Fluid Movementwith Spin Labeling at MR Imaging: Preliminary Results in Normal andPathophysiologic Conditions,” Radiology, 2008; 249; 644-652. Thismethod, however, is employed to image the flow of cerebrospinal fluidflow rather than for MRA applications. Additionally, it uses twoinversion pulses, rather than saturation pulses, and has a very long TI(on the order of 2500 ms) that is incompatible with MRA studies.

It would therefore be desirable to provide a method for non-contrastenhanced MRA that produced images of a patient's vasculature in arelatively short duration of time while maintaining significantdiscrimination of the arteries and substantially suppressing venoussignals. Moreover, it would be desirable to provide a method fornon-contrast enhanced MRA that was insensitive to flow velocities andwas relatively insensitive to patient motion and other imagingartifacts.

SUMMARY OF THE INVENTION

The present invention overcomes the aforementioned drawbacks byproviding a method for non-contrast enhanced magnetic resonanceangiography (“MRA”) that has a short scan time and is insensitive topatient motion, while maintaining significant arterial conspicuity andsubstantial venous signal suppression. This is achieved by employing atwo-dimensional (“2D”) single shot acquisition, or in some embodiments,a 2D multi-shot acquisition having an echo train length sufficientlyshort so as to substantially suppress blood flow-related artifacts.Advantageously, using such a multi-shot acquisition also allows for dataacquisition without requiring cardiac gating.

It is an aspect of the invention to provide a method for non-contrastenhanced MRA that is substantially insensitive to variations in arterialflow velocity or heart rate and that further substantially suppressesunwanted venous signal in a prescribed 2D imaging slice. A method foraccurately depicting normal and diseased arteries despite a wide rangeof flow velocities is provided by synchronizing a quiescent interval(“QI”) to the period of rapid, systolic arterial blood flow so as tomaximize flow-related enhancement into the imaging slice. Additionally,this method is provided by synchronizing data acquisition to occurduring diastole when arterial blood is stationary or flowing relativelyslowly into a prescribed imaging slice. If venous blood flow is flowingin a cranial direction and arterial blood flow in a caudal direction, asis the case for the peripheral arteries, this venous suppression can befurther augmented by the application of saturation pulses prior to theQI. The saturation pulses are applied not only in the imaging slice, butalso in a slab contiguous with and caudal to the imaging slice.Likewise, when venous blood is flowing in a caudal direction andarterial blood in the cranial direction, as is the case in the internaljugular vein and common carotid artery, respectively, the slab isselected to be contiguous with and cranial to the imaging slice. In thismanner, saturated venous spins and unsaturated arterial spins will flowinto the imaging slice over the duration of the QI. Such a methoddiffers fundamentally from previously described methods for suppressingvenous signal since, in prior methods, saturation pulses were applieddirectly before the radio frequency (“RF”) excitation, allowingnegligible time for inflow of saturated venous spins into the imagingslice.

It is another aspect of the invention to provide a method fornon-contrast enhanced MRA that can produce images in a relatively shortduration of time. For example, the entire length of the peripheralarteries can be imaged in 8 minutes with high arterial conspicuity andmarked suppression of venous signal. Since the method uses a single shotacquisition, no ghost artifacts occur and the method is highly resistantto motion artifact. Previously described MRA techniques, including freshblood imaging and 2D time of flight, do not use a single shotacquisition and thus are prone to motion artifacts and longer scantimes. Additionally, the shortened scan time required with the method ofthe present invention allows for a series of 2D images to be obtainedwithin a single breath-hold.

It is another aspect of the invention to provide a method fornon-contrast-enhanced MRA using an MRI system to produce an image thatdepicts a subject's vasculature. The MRI system is directed to perform apulse sequence that includes (i) applying at least one radio frequency(RF) saturation pulse to a selected region in the subject such thatmagnetic resonance signals in the selected region are at least partiallysuppressed; (ii) applying at least one RF saturation pulse to aprescribed imaging slice that is outside of the selected region suchthat magnetic resonance signals in the prescribed imaging slice are atleast partially suppressed; waiting a period of time after (i) or (ii)during which no RF pulses are applied; and (iv) acquire k-space datafrom the prescribed imaging slice after waiting the period of time in(iii) by applying an excitation RF pulse to the prescribed imaging sliceand sampling k-space using a multi-shot acquisition. The period of timein (iii) is sufficiently long to allow saturated vascular spins from theselected region to flow into the prescribed imaging slice and to allowunsaturated vascular spins from outside the selected region to flow intothe prescribed imaging slice. The multi-shot acquisition used in (iv)has an echo train length sufficiently short so as to substantiallysuppress blood flow-related artifacts. An image that depicts theunsaturated vascular spins in the subject's vasculature is thenreconstructed from the acquired k-space data.

The foregoing and other aspects and advantages of the invention willappear from the following description. In the description, reference ismade to the accompanying drawings which form a part hereof, and in whichthere is shown by way of illustration a preferred embodiment of theinvention. Such embodiment does not necessarily represent the full scopeof the invention, however, and reference is made therefore to the claimsand herein for interpreting the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an MRI system that employs the presentinvention;

FIG. 2 is a pictorial representation of a pulse sequence performed bythe MRI system of FIG. 1 when practicing an embodiment the presentinvention;

FIG. 3 is a pictorial representation of a pulse sequence performed bythe MRI system of FIG. 1 when practicing another embodiment the presentinvention;

FIG. 4A is an example of a pulse sequence that can be performed by theMRI system of FIG. 1 when practicing another embodiment the presentinvention; and

FIG. 4B is another example of a pulse sequence that can be performed bythe MRI system of FIG. 1 when practicing another embodiment the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Referring particularly to FIG. 1, the preferred embodiment of theinvention is employed in an MRI system. The MRI system includes aworkstation 110 having a display 112 and a keyboard 114. The workstation110 includes a processor 116 that is a commercially availableprogrammable machine running a commercially available operating system.The workstation 110 provides the operator interface that enables scanprescriptions to be entered into the MRI system. The workstation 110 iscoupled to four servers: a pulse sequence server 118; a data acquisitionserver 120; a data processing server 122, and a data store server 123.The workstation 110 and each server 118, 120, 122 and 123 are connectedto communicate with each other.

The pulse sequence server 118 functions in response to instructionsdownloaded from the workstation 110 to operate a gradient system 124 anda radio frequency (“RF”) system 126. Gradient waveforms necessary toperform the prescribed scan are produced and applied to the gradientsystem 124 that excites gradient coils in an assembly 128 to produce themagnetic field gradients G_(x), G_(y), and G_(z) used for positionencoding MR signals. The gradient coil assembly 128 forms part of amagnet assembly 130 that includes a polarizing magnet 132 and awhole-body RF coil 134.

RF excitation waveforms are applied to the RF coil 134 by the RF system126 to perform the prescribed magnetic resonance pulse sequence.Responsive MR signals detected by the RF coil 134 or a separate localcoil (not shown in FIG. 1) are received by the RF system 126, amplified,demodulated, filtered and digitized under direction of commands producedby the pulse sequence server 118. The RF system 126 includes an RFtransmitter for producing a wide variety of RF pulses used in MR pulsesequences. The RF transmitter is responsive to the scan prescription anddirection from the pulse sequence server 118 to produce RF pulses of thedesired frequency, phase and pulse amplitude waveform. The generated RFpulses may be applied to the whole body RF coil 134 or to one or morelocal coils or coil arrays (not shown in FIG. 1).

The RF system 126 also includes one or more RF receiver channels. EachRF receiver channel includes an RF amplifier that amplifies the MRsignal received by the coil to which it is connected and a detector thatdetects and digitizes the I and Q quadrature components of the receivedMR signal. The magnitude of the received MR signal may thus bedetermined at any sampled point by the square root of the sum of thesquares of the I and Q components:M=√{square root over (I ² +Q ²)},

and the phase of the received MR signal may also be determined:

$\phi = {{\tan^{- 1}\left( \frac{Q}{I} \right)}.}$

The pulse sequence server 118 also optionally receives patient data froma physiological acquisition controller 136. The controller 136 receivessignals from a number of different sensors connected to the patient,such as ECG signals from electrodes or respiratory signals from abellows. Such signals are typically used by the pulse sequence server118 to synchronize, or “gate”, the performance of the scan with thesubject's respiration or heart beat.

The pulse sequence server 118 also connects to a scan room interfacecircuit 138 that receives signals from various sensors associated withthe condition of the patient and the magnet system. It is also throughthe scan room interface circuit 138 that a patient positioning system140 receives commands to move the patient to desired positions duringthe scan.

The digitized MR signal samples produced by the RF system 126 arereceived by the data acquisition server 120. The data acquisition server120 operates in response to instructions downloaded from the workstation110 to receive the real-time MR data and provide buffer storage suchthat no data is lost by data overrun. In some scans the data acquisitionserver 120 does little more than pass the acquired MR data to the dataprocessor server 122. However, in scans that require information derivedfrom acquired MR data to control the further performance of the scan,the data acquisition server 120 is programmed to produce suchinformation and convey it to the pulse sequence server 118. For example,during prescans MR data is acquired and used to calibrate the pulsesequence performed by the pulse sequence server 118. Also, navigatorsignals may be acquired during a scan and used to adjust RF or gradientsystem operating parameters or to control the view order in whichk-space is sampled. And, the data acquisition server 120 may be employedto process MR signals used to detect the arrival of contrast agent in amagnetic resonance angiography (MRA) scan. In all these examples thedata acquisition server 120 acquires MR data and processes it inreal-time to produce information that is used to control the scan.

The data processing server 122 receives MR data from the dataacquisition server 120 and processes it in accordance with instructionsdownloaded from the workstation 110. Such processing may include, forexample: Fourier transformation of raw k-space MR data to produce two orthree-dimensional images; the application of filters to a reconstructedimage; the performance of a backprojection image reconstruction ofacquired MR data; the calculation of functional MR images; thecalculation of motion or flow images, etc.

Images reconstructed by the data processing server 122 are conveyed backto the workstation 110 where they are stored. Real-time images arestored in a data base memory cache (not shown) from which they may beoutput to operator display 112 or a display 142 that is located near themagnet assembly 130 for use by attending physicians. Batch mode imagesor selected real time images are stored in a host database on discstorage 144. When such images have been reconstructed and transferred tostorage, the data processing server 122 notifies the data store server123 on the workstation 110. The workstation 110 may be used by anoperator to archive the images, produce films, or send the images via anetwork to other facilities.

Referring now particularly to FIG. 2, a pulse sequence employed whenpracticing an embodiment of the present invention is pictorially shown.The pulse sequence is cardiac gated, such that the acquisition ofk-space data is timed with respect to the flow of arterial blood.Specifically, the pulse sequence is timed to be played out with respectto the peak of the R-wave 250 in a concurrently acquired echocardiogram(“ECG”). It should be appreciated by those skilled in the art, however,that other methods for cardiac synchronization of the following pulsethan relying on an ECG signal are possible. For example, cardiacsynchronization can be achieved using self-gating techniques that relyon measurement of flow signal or phase. Turning now to the pulsesequence, a slice-selective RF saturation pulse 260 is first played outin the presence of a slice-selective gradient 262 The application ofthis slice-selective RF saturation pulse 260 has the effect ofsuppressing the signals from background tissues as well as those venousspins that are present in the prescribed slice. The flip angle for thisslice-selective RF saturation pulse 260 is typically about 90 degrees;however, larger or smaller flip angles may be desirable in somecircumstances. The slice-selective RF saturation pulse 260 is timed tooccur at a preset time delay (“TD”) after the occurrence of the R-wave250. For example, TD is set to 100 ms.

The duration of time that is allowed to pass after the application ofthe slice-selective RF saturation pulse 260 is herein referred to as the“quiescent interval” (“QI”). This duration of time is specificallytailored to coincide with the rapid inflow of arterial blood into aprescribed imaging slice, and so that the zero line of k-space isacquired during the slow, diastolic inflow of arterial blood into theimaging slice. More particularly, a central portion of k-space issampled during the slow, diastolic inflow of arterial blood into theimaging slice before the peripheral regions of k-space are sampled. Theresult of this is a substantial suppression of flow-based imageartifacts. Put another way, this provides a method that is substantiallyinsensitive to flow velocities in the prescribed image slice. Moreover,the QI allows for a maximal inflow of unsaturated arterial spins intothe imaging slice, such that an improved discrimination of arterialspins is provided in the resultant images. This is even so when thepatient's vasculature is significantly impacted by vascular diseasessuch as peripheral vascular disease (“PVD”). Exemplary values of QI inthis configuration of the pulse sequence are on the order of 260 ms.

After the QI has passed, the pulse sequence proceeds with dataacquisition, which is accomplished, for example, with a single shotbalanced steady-state free procession (SSFP) gradient echo pulsesequence. First, a spectrally selective fat saturation RF pulse 254 isapplied to further suppress unwanted MR signals originating from fattissue. This is subsequently followed by a slice-selective α/2magnetization RF pulse 256 that is played out in the presence of aslice-selective gradient 268, where α is a user selected flip angle. Theslice-selective gradient includes a rephasing lobe 270 that acts tomitigate unwanted phase accruals that occur during the application ofthe slice-selective gradient 268. This portion of the pulse sequenceincludes a slice-selective RF excitation pulse 200 that is played out inthe presence of a slice-selective gradient pulse 202 to producetransverse magnetization in a prescribed slice. The slice-selectivegradient includes a rephasing lobe 204 that acts to mitigate unwantedphase accruals that occur during the application of the slice-selectivegradient 202. After excitation of the spins in the slice, a phaseencoding gradient pulse 206 is applied to position encode the MR signal208 along one direction in the slice. A readout gradient pulse 210 isalso applied after a dephasing gradient lobe 212 to position encode theMR signal 206 along a second, orthogonal direction in the slice. Likethe slice-selective gradient 202, the readout gradient 210 also includesa rephasing lobe 214 that acts to mitigate unwanted phase accruals.

To maintain the steady state condition, the integrals along the threegradients each sum to zero during the repetition time (“TR”) period. Toaccomplish this, a rewinder gradient lobe 216 that is equal inamplitude, but opposite in polarity of the phase encoding gradient 206,is played out along the phase encoding gradient axis. Likewise, adephasing lobe 218 is added to the slice select gradient axis, such thatthe dephasing lobe 218 precedes the repetition of the slice-selectivegradient 202 in the next TR period. As is well known in the art, thereading out of MR signals following the single shot of the RF excitationpulse 200 is repeated and the amplitude of the phase encoding gradient206 and its equal, but opposite rewinder 216 are stepped through a setof values to sample 2D k-space in a prescribed manner. It should beappreciated by those skilled in the art that any number of dataacquisition schemes can be employed to acquire k-space data instead ofbalanced SSFP. For example, spoiled gradient echo, spiral acquisition,or echo planar imaging (“EPI”) pulse sequences can alternatively beutilized.

Referring now particularly to FIG. 3, another configuration of the pulsesequence employed when practicing the present invention is pictoriallyshown. This pulse sequence is also cardiac gated, such that theacquisition of k-space data is timed with respect to the flow ofarterial blood. Specifically, the pulse sequence is timed to be playedout with respect to the peak of the R-wave 250 in a concurrentlyacquired echocardiogram (“ECG”). However, the data acquisition portionof the pulse sequence is preceded by the application of two RFsaturation pulses. By employing RF saturation pulses instead ofinversion recovery pulses, the longitudinal magnetization is alwaysreset to zero prior to the beginning of the QI. This is not necessarilythe case when employing inversion recovery-based methods, as residuallongitudinal magnetization resulting from inadequate inversion mayconfound the subsequently detected MR signals. Moreover, by alwaysresetting the longitudinal magnetization of the slice to zero, the useof a slice-selective RF saturation pulse ensures that the tissue signalremains substantially uniform across different slices despite variationsin the R-R interval due to cardiac arrhythmias. This is not the casewith inversion-recovery based methods.

First, a slice-selective RF saturation pulse 260 is played out in thepresence of a slice-selective gradient 262. The application of thisslice-selective RF saturation pulse 260 has the effect of suppressingthe signals from background tissues as well as those venous spins thatare present in the prescribed slice. The slice-selective RF saturationpulse 260 is timed to occur at a preset time delay (“TD”) after theoccurrence of the R-wave 250. For example, TD is set to 100 ms.

To suppress signals from venous spins that will flow into the prescribedslice, a second, slab-selective RF saturation pulse 264 is played out inthe presence of a slab-selective gradient 266. The slab-selective RFsaturation pulse 264 has the effect of suppressing venous signals in aslab that is contiguous with the prescribed imaging slice. Specifically,when imaging the peripheral arteries, the prescribed slab is chosen tobe caudal to, and contiguous with, the prescribed imaging slice suchthat saturated venous blood flows into the imaging slice in thecaudal-cranial direction while unsaturated arterial spins flow into theimaging slice in the cranial-caudal direction. Similarly, when imaging,for example, the carotid artery, the prescribed slab is chosen to becranial to, and contiguous with, the prescribed imaging slice such thatsaturated venous blood flows into the imaging slice in thecranial-caudal direction while unsaturated arterial spins flow into theimaging slice in the caudal-cranial direction. Exemplary RF excitationslab thicknesses are on the order of 150 mm; however, it will beappreciated by those skilled in the art that other thicknesses can beselected depending on the intended application and subject at hand. Itwill also be appreciated that in some circumstances it may be desirableto shift the RF excitation slab so that it is not contiguous with theimaging slice.

In this alternate configuration of the pulse sequence, the QI is definedas the duration of time occurring after the application of theslab-selective RF saturation pulse 264 and before the fat saturationpulse 254. Like the previously described QI, however, this duration oftime is specifically tailored so that the zero line of k-space isacquired during the slow, diastolic inflow of arterial blood into theprescribed imaging slice. As described above, the result of allowing theQI to pass before data acquisition is a substantial suppression offlow-based image artifacts. Put another way, this provides a method thatis substantially insensitive to flow velocities in the prescribed imageslice. Exemplary values of QI in this configuration of the pulsesequence are on the order of 250 ms. The proceeding data acquisition isplayed out similar to the above-described pulse sequence, which is sodescribed with respect to FIG. 2.

Unlike previous methods that rely on multi-shot acquisitions, therepetition time of the pulse sequences employed when practicing thepresent invention are significantly shorter than, say, the 30 mstypically required of such multi-shot sequences. In this manner, theoverall scan time is less than the several seconds required of previousmethods. This allows the method of the present invention to acquire thedesired image data without becoming significantly sensitive to subjectmotion. Additionally, the shortened data acquisition period provided bythe present invention allows image data to be acquired during only aportion of arterial flow, such that variations in flow velocity duringthe cardiac cycle do not confound the acquired MR signals.

In some embodiments, a multi-shot acquisition can be implemented,however, to achieve an echo train length that is sufficiently short soas to substantially suppress blood flow-related artifacts. Example ofpulse sequences that implement a multi-shot acquisition are illustratedin FIGS. 4A and 4B. This pulse sequence includes the application of oneor more RF saturation pulses 402 to the imaging slice, and theapplication of one ore more RF saturation pulses 404 to a region outsideof the imaging slice. In doing so, magnetic resonance signals in theimaging slice are at least partially suppressed, as are magneticresonance signals in the region outside of the imaging slice. A periodof time 406 is then allowed to pass, during which no RF pulses areapplied. The period of time 406 is sufficiently long to allow saturated,undesired vascular spins from the region outside the imaging slice toflow into the imaging slice, and to allow unsaturated, desired vascularspins from outside the imaging slice to flow into the imaging slice. Asshown in FIG. 4A, the in-slice RF saturation pulses 402 can be appliedbefore the out-of-slice RF saturation pulses 404, or as illustrated inFIG. 4B, the order in which these RF saturation pulses 402, 404 areapplied can be switched.

Data is then acquired from the imaging slice using a multi-shotacquisition 408. For instance, an excitation RF pulse is applied to theimaging slice and k-space data is acquired by sampling k-space using amulti-shot acquisition scheme. Examples of multi-shot acquisitions thatcan be used include, but are not limited to, steady-state freeprecession (“SSFP”) sequences, including balanced SSFP (“bSSFP”)sequences. The multi-shot acquisition 408 has an echo train length(“ETL”) that is sufficiently short so as to substantially suppress bloodflow-related artifacts. Advantageously, using such a multi-shotacquisition results in being able to acquire k-space data withoutcardiac gating. The k-space data is acquired by sampling k-space alongone or more k-space trajectories. These k-space trajectories can be, forexample, radial, spiral, or Cartesian trajectories.

The pulse sequence can be repeated to acquire k-space data from multipledifferent slice locations. These multiple images can be processed toproduce an angiographic image, as is known in the art. Additionally, thepulse sequence can be repeated to acquire k-space data from a pluralityof different segments of k-space in each repetition. Images can then bereconstructed using only a subset of these different segments.

Motion-related artifacts, such as blood flow-related artifacts, can beidentified or otherwise detected in the k-space data and then correctedfor before reconstructing images. As one example, when k-space data isacquired as a plurality of different segments, as described above, thesedata segments can be analyzed to detect whether motion-related artifactsare present in the acquired data. Similarly, navigator data can beseparately acquired and used to detect whether motion-related artifactsare present in the k-space data. When such artifacts are present, thek-space data can be corrected using motion correction techniques knownin the art.

The present invention has been described in terms of one or morepreferred embodiments, and it should be appreciated that manyequivalents, alternatives, variations, and modifications, aside fromthose expressly stated, are possible and within the scope of theinvention. For example, it should be appreciated by those skilled in theart that a plurality of different saturation pulses may be employedprior to the beginning of the QI, with an appropriate change in QI. Forexample, as more saturation pulses are added, QI may decrease down toabout 100 ms. Additionally, the specific order of the RF saturationpulses can be altered, such that a slab-selective saturation pulse isapplied prior to a slice-selective saturation pulse, withoutsignificantly changing the resultant image contrast.

The invention claimed is:
 1. A method for non-contrast-enhanced magneticresonance angiography (MRA) using a magnetic resonance imaging (MRI)system to produce an image that depicts a subject's vasculature, thesteps of the method comprising: a) directing the MRI system to perform apulse sequence that includes: i) applying at least one radio frequency(RF) saturation pulse to a selected region in the subject such thatmagnetic resonance signals in the selected region are at least partiallysuppressed; ii) applying at least one RF saturation pulse to aprescribed imaging slice that is outside of the selected region suchthat magnetic resonance signals in the prescribed imaging slice are atleast partially suppressed; iii) waiting a period of time after at leastone of steps i) and ii) during which no RF pulses are applied, theperiod of time being sufficiently long to allow saturated vascular spinsfrom the selected region to flow into the prescribed imaging slice andto allow unsaturated vascular spins from outside the selected region toflow into the prescribed imaging slice; iv) acquire k-space data fromthe prescribed imaging slice after waiting the period of time in stepiii) by applying an excitation RF pulse to the prescribed imaging sliceand sampling k-space using at least one of a single-shot acquisition anda multi-shot acquisition having an echo train sufficient in length so asto suppress blood flow-related artifacts; and b) reconstructing from thek-space data acquired in step a)iv), an image that depicts theunsaturated vascular spins in the subject's vasculature.
 2. The methodas recited in claim 1 in which step a)iv) includes acquiring k-spacedata from one segment of k-space, and step a) is repeated a plurality oftimes to acquire k-space data from different segments of k-space.
 3. Themethod as recited in claim 2 in which step b) includes reconstructingfrom the acquired k-space data, a plurality of images that depict theunsaturated vascular spins in the subject's vasculature, each of theplurality of images corresponding to a different slice location.
 4. Themethod as recited in claim 3 further comprising producing anangiographic image from the plurality of images.
 5. The method asrecited in claim 2 in which step b) includes reconstructing the imagefrom k-space data corresponding to only a subset of the differentsegments of k-space.
 6. The method as recited in claim 2 furthercomprising detecting whether motion-related artifacts are present in theacquired k-space data by analyzing the k-space data associated with thedifferent segments of k-space.
 7. The method as recited in claim 6further comprising correcting the acquired k-space data whenmotion-related artifacts are present in the acquired k-space data. 8.The method as recited in claim 6 in which the motion-related artifactsare flow-related artifacts.
 9. The method as recited in claim 1 furthercomprising acquiring navigator data and detecting whether motion-relatedartifacts are present in the acquired k-space data by analyzing thenavigator data.
 10. The method as recited in claim 9 further comprisingcorrecting the acquired k-space data when motion-related artifacts arepresent in the acquired k-space data.
 11. The method as recited in claim1 in which step a)iv) includes acquiring k-space data by samplingk-space along at least one k-space trajectory.
 12. The method as recitedin claim 11 in which the at least one k-space trajectory is at least oneof a radial trajectory, a spiral trajectory, and a Cartesian trajectory.13. The method as recited in claim 1 in which step a)i) is performedbefore step a)ii), and in which the period of time in step a)iii) occursafter step a)ii) is performed.
 14. The method as recited in claim 1 inwhich step a)ii) is performed before step a)i), and in which the periodof time in step a)iii) occurs after step a)i) is performed.
 15. Themethod as recited in claim 1 in which step a)iv) includes acquiringk-space data by sampling k-space using a multi-shot steady-state freeprecession (SSFP) acquisition.
 16. The method as recited in claim 15 inwhich the multi-shot SSFP acquisition is a balanced SSFP (bSSFP)acquisition.
 17. The method as recited in claim 1 in which the period oftime in step a)iii) is between 100 and 1000 milliseconds.
 18. The methodas recited in claim 1 in which the period of time in step a)iii) isselected such that step a)iv) occurs during slow diastolic inflow ofunsaturated vascular spins associated with arterial blood into theprescribed imaging slice.
 19. The method as recited in claim 1, whereinthe at least one RF saturation pulse has a flip angle of 90 degrees.